My KS campaign is on its last day. I have been active on the KBoards throughout, and made some conclusions already. I'll check back in when I hear back, but here is what I think so far. I want to stress that these are my opinions based on what I've seen. I don't have any secret knowledge from the KS editors.
Campaign stats have little to do with what gets accepted. Books with high stats get rejected and books with low stats can be accepted. The nominations have a different purpose. They generate a reader base for advance copies to generate reviews before the book is released.
Hot and Trending is not all about number of nominations. Time spent on page, ratings, and comments make a huge difference. I had one day when I only had 22 page views, but stayed H&T all 24 hours. This was after I got my old fanfic readers involved in the campaign. They are likely to read the entire excerpt and make comments, whereas friends and family on facebook are likely to hit the nomination button and leave. Quality of nominations matters more than quantity.
As for how editors make selections, I think it is like any publisher anywhere. One book I nominated that was not selected surprised me. It was very well written and engaging, but perhaps it was too similar to another project already accepted. Or perhaps they needed no more books in that genre at that time. Hard to tell. Publishers have criteria to follow other than good writing.
I read a ton of campaigns through the month. Even when my nominations were full, I wanted to see what was out there. Many books not selected were, in my opinion, not ready for publication. Some were getting there, making me feel that if the authors had a couple more books under their belt, they would have a better chance. Of the books not selected, one read like a bad translation into English, one used words that did not mean what they thought, several did not have strong voice, and several had characters that all sounded the same. There were also some that began with info dumping or character study.
I would have to say that submitting to Kindle Scout is like submitting to any other publisher. Nothing about it makes it easier, or gives you a better chance. A successful campaign and self promotion won't get a subpar book published. I do believe that all books submitted get eyes on them, which is nothing to sniff at, but you will not get feedback. If you are going to throw your hat in the ring, same rules apply as anywhere else. Polish it until it shines, get some crit, get some editing, get a decent cover, and write a better description than I did.
Once again, these are no more than opinions I have drawn through my experience. I'll be back in 7-12 days to cheer or cry, and tell you my campaign stats.